Car-mounted navigation devices have been provided in the past for guiding the driver of an automobile along the optimum route from a departure point to a destination point. A conventional navigation device has a CD-ROM, IC card, or other map data storage device in which map data are recorded; a display device; and a gyroscope, a GPS (Global Positioning System), a speed sensor, and other vehicle movement detection devices and the like for detecting the current position and the current bearing of the vehicle. Map data that include the current position of the vehicle are read from the map data storage device, a map image of the vicinity of the vehicle's position is created on the display device on the basis of the map data, and by superimposing a vehicle position marker (location) on the display screen, scrolling the map image according to the movement of the vehicle, or keeping the map image fixed on the screen while moving the vehicle position marker, the location in which the vehicle is currently traveling can be instantly recognized.
Such a vehicle-mounted navigation device is usually equipped with a route guidance function to ensure that the driver can easily travel to the desired destination point without mistaking the route. In this route guidance function, the map data are used to calculate a simulation by the Dijkstra method or the like and search for the lowest-cost route that links the departure point to the destination point, the searched route is stored in advance as a guidance route, and the guidance route is displayed on the map image in bold and in a different color than the other roads. When the vehicle approaches within a certain distance from an intersection at which the course on the guidance route is to be changed, an arrow indicating the course is displayed at the intersection at which there is to be a course change on the map image, whereby the driver can easily recognize the optimum route to the destination point.
The vehicle-mounted navigation device described above is a standalone navigation device that has map data or a route search function, but such a navigation device must be provided with all the functions necessary for navigation, which increases the size and cost of the device. Recent developments in communication and information processing techniques have led to the widespread use of so-called communication-type navigation systems in which functionality for communicating via a network is added to the vehicle-mounted navigation device to enable data communication with a route search server and to acquire guidance route data or map data. A system in which a mobile telephone is used as a navigation terminal has also been implemented as a pedestrian navigation system.
The map data used for route searching in a pedestrian navigation system or a car navigation system are referred to as road network data. When the road network is composed of roads A, B, and C as shown in FIG. 20, for example, the end points, intersection points, turning points, and other points of roads A, B, and C are designated as nodes; roads linking the nodes are indicated by directional links; and the road network data are composed of node data (node latitude/longitude), link data (link numbers), and link costs (distance between links or time required to travel to a link) in the form of link cost data. Specifically, in FIG. 20, the symbols — and ⊚ indicate nodes, wherein the ⊚ indicates an intersection of roads. Directional links between nodes are indicated by arrow lines (solid lines, dashed lines, chain double-dashed lines). Links in the upstream and downstream directions of the roads are present, but only links in the direction of the arrows are shown in FIG. 20 to simplify the diagram.
When the data of such a road network is route-searched as a database for route searching, a link connected from the node of the departure point to the node of the destination point is traced, the link cost is stored, and the route having the smallest stored link cost is searched and used for guidance. Specifically, in cases in which a route search is performed using node AX in FIG. 20 as the departure point and node CY as the destination point, sequential tracing and accumulation of the link cost are performed for the link to node CY in which road A is traveled from node AX and a right turn into road C is made at the second intersection point, and the route having the smallest cumulative value for the link cost is searched and used for guidance. Other routes from node AX to node CY are not shown in FIG. 20. However, other routes actually exist, and routes whereby it is possible to reach node CY from node AX are therefore searched in the same manner, and the route having the smallest link cost among the searched routes is determined to be the optimum route. This technique is in accordance with the publicly known technique known as the Dijkstra method.
The road network data in the vehicle-mounted navigation system are composed only of roads that can be traveled by automobile, and there is no need for pedestrian road network data on pedestrian-only roads or roads inside parks or in front of train stations where vehicle entry is prohibited. In a navigation system for pedestrians, data for road networks that can be traveled by automobile are included in addition to the above-described pedestrian road network data for pedestrian-only roads or roads inside parks or in front of train stations where vehicle entry is prohibited, but there is no need for network data for highways and other roads on which walking is prohibited.
In a navigation system in which a mobile telephone is used as a navigation terminal, since there are also situations in which a user places the mobile telephone inside an automobile and uses the mobile telephone in the same manner as a car navigation system, the route search server is provided with road network data for performing automobile route searches as well as pedestrian road network data for performing pedestrian route searches, and the appropriate road network data are used for the route search according to the transportation mode (walking or riding) specified by the user as a condition for the route search. In the pedestrian navigation system, since it is often the case that transportation facilities are also used by the pedestrian, the route search server is furthermore provided with network data for lines in a transportation facility and running time data for trains, commuter trains, buses, and the like that move on each line, and the route search is performed using line network data and running time data when a transportation facility is specified as the transportation mode.
In such a navigation system, the guidance route is displayed on the map in the navigation terminal, and important buildings and other structures on the map are displayed to make the current position or the direction of travel easier to recognize. The route search server therefore stores a database of map data for display that include building data in addition to the road network data used for route searching. Such road network data or map data are composed of unit data (mesh data) that are divided into prescribed sizes by latitude and longitude, and the route search server transmits the optimum guidance route data obtained as a result of the route search to the navigation terminal, and transmits map data that include the current position, i.e., eight units of mesh data that surround mesh data at the center that include the current position, to the navigation terminal on the basis of current position information (latitude and longitude) received from the navigation terminal.
When the guidance route data and the map data are received from the route search server, the navigation terminal temporarily stores these data in a storage means, and presents the guidance route, a current position marker indicating the current position of the navigation terminal, and a prescribed range of the map that includes the current position, to VRAM and displays these items in the display means. When the current position changes and the map data are inadequate, the navigation terminal requests map data from the route search server and receives a transmission of the needed map data. The road network data or the map data are accumulated in the database in the form of vector data, and the navigation terminal can perform prescribed computation of the data as needed to enlarge or rotate the map or the guidance route.
The method for transmitting a map and displaying the map in a terminal device in this manner is not limited to a navigation system for performing route searching and route guidance, and is used in the same manner in a map display system for transmitting a current position or desired position information and receiving a transmission of map data. Attempts have been made to realistically convey the layout of the vicinity to the user in the form of an image that approximates the actual landscape by displaying the map as a bird's-eye view in which important buildings on the map are indicated as three-dimensional representations when a navigation terminal or other terminal device displays a map.
For example, a map display device is disclosed in Patent Reference 1 (Japanese Laid-open Patent Application No. 2001-27534) in which even the shapes of building walls are drawn and displayed in three dimensions. The map display device disclosed in Patent Reference 1 is aimed at making it easier to identify structures and recognize the correspondence to the actual landscape in a map display device that is used as a vehicle navigation device, and is configured so as to display a set window frame shape and wall surface color according to the type of building displayed in three dimensions. The number of window frames in the display is the same as the number of floors of the building, and the roof surface of the building at the destination point is colored dark red. Specifically, this map display device is a map display device for displaying roads, structures, and the like in three dimensions in a display means on the basis of map data that are stored in a map data storage means, wherein the structures are displayed in the display means along with modifications according to characteristics of the structures.
In order to create a display such as the one described above, the map data storage device in the map display device disclosed in Patent Reference 1 is composed of a DVD-ROM, a hard disk, a CD-ROM, or other high-capacity storage medium, and a playback device for retrieving the data stored in the storage medium. A three-dimensional digital (road) map database that includes road map data and data and the like for creating a three-dimensional display of various types of structures and the like (government administration offices, banks, schools, station buildings, airports, hotels, buildings, various types of facilities, and other structures) is recorded in the storage medium. Text information for displaying the names of intersections, cities, towns, villages, and the like is also recorded in the storage medium. The data for creating a three-dimensional display of structures in the road map data are composed of position data and polygon data and other data that denote the planar shape of buildings, height (number of floors) data, and other characteristics. Type data that indicate the type of structure, and text information for displaying the names of structures are recorded in the storage medium.
Patent Reference 2 (Japanese Laid-open Patent Application No. 9-134122) discloses a vehicle map display device for displaying a representation of the height direction of a bird's-eye view for land that is at or above a prescribed height in the map display, rather than a building display. The vehicle map display device disclosed in Patent Reference 2 displays a realistic bird's-eye view through simple processing. Specifically, this vehicle map display device reads the range of road map data to be displayed from the map storage memory, detects the elevation information included in the read road map data, determines whether points on the road map are at an elevation equal to or higher than a reference elevation h (meters), and sets the elevation of regions lower than the reference elevation of h meters to zero meters. Only the map region that has an elevation equal to or higher than the reference elevation of h meters is thereby displayed three-dimensionally when the bird's-eye view is displayed, and map regions that are lower than the reference elevation of h meters are displayed at a constant elevation. The map display is therefore easier to read, and the draw speed of the map is enhanced.
In general, when buildings on the map are displayed as three-dimensional representations in a bird's-eye view such as the one shown in FIG. 21, there is a need to process portions that are observed from behind buildings; i.e., so-called hidden surface removal processing is needed to remove the image data of portions of the rear buildings 23 or 25 that are blocked by the images of the front buildings 22 or 24. For example, Patent Reference 3 (Japanese Laid-open Patent Application No. 2003-263102) discloses a map display device in which hidden surface removal processing is performed by a method referred to as a Z buffer method.
The map display device disclosed in Patent Reference 3 creates a three-dimensional map in which structural elements of the map are arranged on the basis of a map storage means. Affine transformation, perspective conversion processing, and other prescribed processing of the three-dimensional map are then performed, after which hidden surface removal processing by a Z buffer method or other method is performed to create a projection drawing obtained when the abovementioned three-dimensional map is viewed from a certain perspective. Information relating to the map components displayed in the projection drawing is then read from an information storage means, after which a display region in the projection drawing of the map components displayed in the projection drawing is detected. The detected display region is then used as a basis for setting a display region of the related information on the projection drawing. The display region of the related information on the projection drawing is set for each map component having related information that is displayed in the projection drawing, and is also set each time there is a change in the displayed region of the map component on the projection drawing.    [Patent Reference 1]: Japanese Laid-open Patent Application No. 2001-27534 (FIGS. 1 and 6, and paragraphs [0015] and [0037] through [0045])    [Patent Reference 2]: Japanese Laid-open Patent Application No. 9-134122 (FIGS. 1 and 2, and paragraphs [0007] and [0013] through [0019])    [Patent Reference 3]: Japanese Laid-open Patent Application No. 2003-263102 (FIG. 29, and paragraphs [0030] through [0034], [0054], and [0055])